1. Field of the Invention
The present invention relates to a semiconductor film crystallization method, a manufacturing method for a semiconductor device, and a laser irradiation apparatus. More specifically, the invention relates to a semiconductor film crystallization method, manufacturing method for a semiconductor device, and laser irradiation apparatus in which the generation of in-plane variations in the quality of a semiconductor film which has been crystallized can be suppressed.
2. Description of the Related Art
In recent years, extensive research has been conducted on laser crystallization methods used to crystallize a semiconductor film (for example, an amorphous semiconductor film) formed over a glass substrate through irradiation of the semiconductor film with a laser beam.
Crystallization of a semiconductor film is performed in order to increase carrier mobility through crystallization of the semiconductor film. The crystallized semiconductor film is used, for example, in a thin film transistor (hereinafter referred to as a TFT). For example, when a semiconductor film formed over a glass substrate has been crystallized, an active matrix display device (for example, a liquid crystal display device or an organic EL display device) can be manufactured through formation of a TFT for use in a pixel and TFT for use in a driver circuit, using the semiconductor films.
Methods for crystallizing a semiconductor film, other than the laser crystallization method, include a thermal annealing method which uses an annealing furnace and a rapid thermal annealing method (RTA method). However, these methods require processing at a high temperature of 600° C. or more. For this reason, use of a quartz substrate that can withstand processing at high temperature becomes necessary and causes manufacturing costs to increase. In comparison, with the laser crystallization method, absorption of heat can be restricted to absorption by the semiconductor film only, and a semiconductor film can be crystallized without any such increase in the temperature of the substrate. Because of this, a material with low heat resistance, such as glass or plastic, can be used for the substrate. As a result, an inexpensive glass substrate that can be easily processed with a large area can be used, and the production efficiency of the active matrix display device increases considerably.
Conventionally, a method using an excimer laser which is a pulsed laser has been used as a laser crystallization method. Because the wavelength of an excimer laser lies in the ultraviolet region, the laser can be efficiently absorbed by silicon and heat can be selectively applied to the silicon. When an excimer laser is used, a laser beam, for example, a laser beam with a rectangular cross section (for example, a rectangular cross section with an area of 10 mm×30 mm), emitted by a laser oscillator is processed by an optical system into a laser beam with a linear cross section (for example, a linear cross section with an area of several hundreds of micrometers by 300 mm). Then, the linearly processed laser beam irradiates the semiconductor film while scanning in relation to the semiconductor film, whereby the whole of the semiconductor film is crystallized sequentially. With the direction in which the beam spot is being scanned being perpendicular relative to the beam spot, crystallization efficiency increases.
In comparison, in recent years, the technology for manufacturing a semiconductor film with crystals of much larger grain size than crystals of a semiconductor film crystallized by an excimer laser has been developed, in which the semiconductor film is irradiated at a linear beam spot with a CW laser or a pulsed laser that has an oscillating frequency (repetition frequency) of 10 MHz or more processed into a laser beam with a linear cross section, scanning in relation to the semiconductor film. When the semiconductor film with crystals of large grain size is used in the channel region of a TFT, energy barriers against carriers (electrons or holes) decrease because fewer grain boundaries exist in the direction of the channel. As a result, the manufacture of a TFT that has a mobility of several hundreds of cm2/(V·s) becomes possible. (For an example, refer to Patent Document 1: Japanese Published Patent Application No. 2003-332236 (Paragraph 4)).
FIG. 37 is a diagram used to explain a first conventional example of a crystallization method which uses a laser beam 801 emitted by a continuous wave laser or mode-locked laser for crystallization of a semiconductor film 802 formed over a substrate 800. In this example, the laser beam 801 is emitted in a direction perpendicular to the substrate 800 and scans relative to the substrate 800 in a direction along A-B in the diagram. Part of the laser beam 801 passes through the semiconductor film 802, reflects off the lower surface 800a of the substrate 800, and interferes with the incoming laser beam 801 at the semiconductor film 802. Because the thickness of the substrate 800 varies with location, the incident light and reflected light of the laser beam 801 are strengthened and weakened by each other, depending on the location. As a result, in-plane variations in the properties of the crystallized semiconductor film 802 are generated.
FIG. 38 is a diagram used for explaining a second conventional example of a crystallization method which uses the laser beam 801 for crystallization of the semiconductor film 802. In this example, the laser beam 801 irradiates the substrate 800 at a diagonal. The diagonal direction of the laser beam 801 is a direction along the scanning direction A-B of the laser beam 801.
In this example, although the incident light and reflected light of the laser beam 801 do not interfere, crystallization conditions differ, depending on the scanning direction of the laser beam 801. That is, when the laser beam 801 scans relative to a direction from B to A in the diagram, the semiconductor film 802 is irradiated with incident light after being irradiated with reflected light. In comparison, when the laser beam 801 scans relative to a direction from A to B in the diagram, the semiconductor film 802 is irradiated with reflected light after being irradiated with incident light. Because the incident light and reflected light of the laser beam 801 differ in intensity, there will be two regions in the semiconductor film 802, each crystallized under different conditions. The properties of each of these two regions differ from those of the other.